Cathodic activation of stainless steel

ABSTRACT

PROCESS FOR PRODUCTION OF STAINLESS STEEL ARTICLES SUCH AS, FOR EXAMPLE, STAINLESS STEEL WIRES, STRIPS, SHEETS AND THE LIKE BY ACTIVATING THE SURFACE OF A STAINLESS STEEL SUBSTRATE BY CATHODIC TREATMENT IN AN AQUEOUS ACID BATH AT HIGH CURRENT DENSITY FOLLOWED BY ELECTROPLATING THE ACTIVATED SURFACE TO PROVIDE THE DESIRED LAYER OR LAYERS OF ELECTROPLATE.

United States Patent Oflice Patented Apr. 4, 1972 3,654,099 CATHODICACTIVATION F STAINLESS STEEL Eric De Bruyne, Deerlijk, Belgium, assignorto N.V. Bekaert S.A., Zwevegem, Belgium No Drawing. Filed June 10, 1970,Ser. No. 45,261 Claims priority, application Great Britain, June 20,1969, 31,369/69 Int. Cl. C23b 5/62 US. Cl. 204-29 28 Claims ABSTRACT OFTHE DISCLOSURE BACKGROUND OF THE INVENTION The cleaning of stainlesssteel prior to electroplating involves a difiiculty in that a thin,transparent and tenacious oxide film is present on the metal. This filmhas the property of reforming quickly on exposure to air or otheroxidizing conditions even after it has once been removed. The presenceof this film prevents adherent coatings of metals being applied byelectroplating, and it is necessary to remove the film beforeelectroplating. In view of the risk of rapid reforming of the oxidefilm, it is then usual to apply a first layer of the electroplated metalimmediately after removal of the film and before the film can commenceto reform, and subsequently to apply a second layer of the desiredelectroplated metal.

The removal of the oxide film from stainless steel prior toelectroplating is generally termed activation. Many methods have beenproposed for the activation of stainless steel. Some such methodsinclude cathodic or anodic treatments followed by immediate applicationof a protective so-called strike layer on to which the desiredelectroplated layer or layers can subsequently be applied. In othermethods, the treatment is such that the oxide film is removed and astrike layer is simultaneously applied; this can for example be achievedby means of a cathodic treatment with certain nickelchloride/hydrochloric acid baths. In all these known methods, a strikelayer which may for example be of nickel, zinc, cobalt or cadmium isapplied to protect the stainless steel after removal of the oxide filmand the desired electroplated layer or layers are subsequently applied.The strike layer is itself applied by electroplating, and thus at leasttwo electroplating steps are involved.

SUMMARY OF THE INVENTION The present invention is based upon thediscovery that it is possible rapidly to activate stainless steel and toavoid the necessity of applying a strike layer by cathode treatment ofthe stainless steel in an aqueous acid bath at high current density.

According to the present invention there is provided a process for theproduction of an electroplated stainless steel article which comprisesactivating the surface of a stainless steel substrate by cathodictreatment in an aqueous acid bath at high current density, followed byelectroplating the activated surface to provide the desired layer orlayers of electroplate.

Following activation under such conditions, the desired electroplatedlayer or layers may be applied to the stainless steel without the needfor application for an intermediate protective strike layer.

The aqueous acid bath used for activation in accord ance with thepresent invention may contain mineral acids such as for example H I-lCl,H PO HBF HF and NH SO H. The acid will advantageously be present in thebath in a concentration to provide a pH of less than about 4. Thecurrent density used during activation is preferably at least 10 a./dm.and is advantageously from 10 to 300 a./dm. The treatment isconveniently carried out using DC electricity with the stainless steelarticle to be coated as cathode and an anode of any conductive materialwhich does not release metal ions into the bath.

The residence time of the stainless steel article to be coated in thebath is generally not critical and can vary within wide limits. Theresidence time may however, for example, be in the range of from 0.1second to 1 minute, and the fact that such short residence times arepossible makes the process according to the invention especially usefulfor the treatment of stainless steel articles such as wires, strips,sheets and the like. Such articles can with advantage be tretaed bycontinuous passage through an activating bath, the articles beingactivated on the run as they pass through the bath.

Following activation, the desired electroplated layer or layers can beapplied to the stainless steel articles by any convenient method withoutapplication of an intermediate strike layer. Again with articles such aswires, strips, and sheets, the electroplating is with advantage effectedduring continuous passage of the article through an electroplating bathor baths. The electroplating baths may for example be aqueous acidbaths, advantageously with a pH not greater than 5. Using such baths,electroplated layers of metals such as for example zinc, iron, cadmium,nickel, lead, manganese, copper, silver and gold can be applied. Ifdesired, aqueous alkaline electroplating baths may also be used; copperand zinc are examples of metals which can conveniently be thus applied.

Additives such as for example brightening agents, may be incorporatedinto the electroplating baths as required.

Before activation in accordance with the present invention, thestainless steel may require cleaning. This is particularly so where thesurface of the stainless steel article to be treated is very impure.Cleaning may thus for example comprise an anodic treatment in diluteaqueous solutions of hydrochloric, sulphuric or phosphoric acid. Thecurrent density and treatment time will depend upon the degree ofimpurity of the stainless steel surface. The more impure the surface,the more total current (i.e. current density x time) will in general berequired to prepare the surface for cathodic activation in accordancewith the invention. Current densities of from 1 to a./dm. and treatmenttimes of from 0.1 second to 1 minute may for example be used withadvantage. The anodic cleaning treatment may be combined with othercleaning treatments e.g. an initial simple immersion of the stainlesssteel article in aqueous acid or alkaline baths.

By the use of processes according to the invention, it is possible toapply on to stainless steel articles electroplated layers of metalswhich are of good quality and which are very firmly bonded to thestainless steel articles.

EXAMPLES The following examples illustrate the invention:

EXAMPLE 1 A stainless steel wire, diameter 3 mm., is continuously passedinto and through a bath containing an aqueous solution of 200 g./l.sulphuric acid. The wire is cathodically treated in the bath at acurrent density of 30 a./dm. with a lead anode, the residence time ofthe wire in the bath being one second.

After withdrawal from the bath, the wire is fed into a second bathcontaining 200 g./l. CuSO -7H O and 200 g./l. H 80 The wire iscathodically treated in this second bath at a current density of 35a./dm. A copper coating of very good quality is thereby electroplated onto the stainless steel wire. This copper coating has very good adhesionto the stainless steel and no clefts occur when the wire is subsequentlybent.

EXAMPLE 2 A stainless steel wire, diameter 3 mm., is passed continuouslyinto and through a bath containing an aqueous solution of 150 g./l.phosphoric acid. The Wire is cathodically treated in the bath at acurrent density of 20 a./dm. with a carbon anode, the residence time ofthe wire in the bath being 0.5 second.

After withdrawal from the bath, the wire is fed into a second bathcontaining 100 g./1. Pb(BF 3O g./l. HBF and 30 g./l. H BO The wire iscathodically treated in this second bath at a current density of 10a./dm. The lead layer deposited is of very good quality and shows goodadhesion to the stainless steel.

EXAMPLE 3 A stainless steel wire, diameter 3 mm., is passed continuouslyinto and through an aqueous solution of 90 g./l. H 80 and 60 g./l. HCl.The wire is anodically treated in the bath at a current density of 10a./dm. the residence time of the wire in the bath being one second. Thewire is subsequently subjected to a cathodic treatment followed bycathodic electroplating as described in Example 1 or Example 2.

EXAMPLE 4 Stainless steel wire is first immersed in an aqueous bathcontaining 100 g./l. of hydrochloric acid at a temperature of 50 C. forfive seconds. The wire is next anodically treated in an aqueous bathcontaining 50 g. /l. of sulphuric acid for three seconds at a currentdensity of 20 a./dm. The wire is next rinsed in water, and is thencathodically treated in an aqueous bath containing 150 g./l. of sulphuric acid for six seconds at a current density of 20 a./dm. The wireis afterwards rinsed again with water and is finally electroplated in abath containing the following:

Cu=40 g./1. KCN=with 5 to 10 g./l. free KCN Na COg=l g./l.

The electroplating is effected with the stainless steel wire as cathodeat a current density of 10 a./dm. and a temperature of 50 C.

EXAMPLE The process of Example 4 is repeated, the wire however beingfinally electroplated in a bath containing the following:

KCN=3.5 concentration of ZniO.2 Na CO g./l.

The electroplating is effected with the stainless steel wire as cathodeat a current density of a./dm. and a temperature of 40 C.

While the invention has been described with respect to certain preferredexamples which give satisfactory results, it will be understood by thoseskilled in the art after un derstanding the principle of the invention,that various changes and modifications may be made without departingfrom the spirit of the invention.

What is claimed is:

1. A process for the production of an electroplated stainless steelarticle which comprises activating the sur- 4 face of a stainless steelsubstrate by cathodic treatment in an aqueous acid bath at a currentdensity of not less than 10 a./dm. and then electroplating saidactivated surface to provide the desired layer or layers ofelectroplate.

2. A process as claimed in claim 1, wherein the aqueous acid has a pH ofless than about 4.

3. A process as claimed in claim 1, wherein the current density used foractivation is at least 10 a./dm.

4. A process as claimed in claim 1, wherein the current density is from10 to 300 a./dm.

5. A process as claimed in claim 1, wherein the anode used to eifect thecathodic treatment of the stainless steel is of a material which doesnot release metal ions to the acid bath.

6. A process as claimed in claim 5, wherein the residence time of thearticle to be treated in the acid bath is from 0.1 second to 1 minute.

7. A process as claimed in claim 1, wherein the substrate treated is inthe form of a wire, strip or sheet.

8. A process as claimed in claim 7, wherein the substrate is treated bycontinuous passage through the acid bath.

9. A process according to claim 8, wherein the electroplating iseffected in an aqueous alkaline electroplating bath.

10. A process as claimed in claim 1, wherein the electroplating iseffected in an aqueous acid electroplating bath.

11. A process as claimed in claim 10, wherein the electroplating iseifected at a pH of not greater than 5.

12. A process as claimed in claim 10, wherein the electroplated layercomprises a metal selected from the group consisting of zinc, iron,cadmium, nickel, lead, manganese, copper, silver and gold.

13. A process as claimed in claim 1, wherein the elec troplating iseffected in an aqueous alkaline electroplating bath.

14. A process as claimed in claim 13, wherein the electroplated layercomprises copper.

15. A process as claimed in claim 13, wherein the electroplated layercomprises zinc.

16. A process as claimed in claim 1 wherein the cathodic treatment iseifected in an aqueous solution of a mineral acid.

17. A process as claimed in claim 16 wheerin the mineral acid isselected from the group consisting of H H3PO4, HBF4, and NHZSOQH- 18. Aprocess as claimed in claim 16, wherein the aqueous acid has a pH ofless than about 4.

19. A process as claimed in claim 16, wherein the current density usedfor activation is at least 10 a./dm.

20. A process as claimed in claim 1, wherein the substrate is subjectedto a cleaning step before the cathodic treatment.

21. A process as claimed in claim 20, wherein the cleaning step iseffected by anodic treatment in dilute aqueous solutions ofhydrochloric, sulphuric or phosphoric acid.

22. A process as claimed in claim 21, wherein the anodic treatment iscombined with an initial cleaning treatment comprising immersion of thesubstrate in an alkaline bath.

23. A process as claimed in claim 21,, wherein the anodic treatment iseifected for a period of from 0.1 seconds to 1 minute.

24. A process as claimed in claim 23, wherein the anodic treatment iscombined with an initial cleaning treatment comprising immersion of thesubstrate in an alkaline bath.

25. A process as claimed in claim 21, wherein the anodic treatment iseifected at a current density of from 1 to a./dm.

26. A process as claimed in claim 25, wherein the anodic treatment iseifected for a period of from 0.1 second to 1 minute.

27. A process as claimed in claim 25, wherein the anodic treatment iscombined with an initial cleaning 5 6 treatment comprising immersion ofthe substrate in an 2,543,545 2/1951 Faust et a1 20455 R alkalme bath-2,791,554 5/1957 Winters 204-55 R 28. Electroplated stainless steelarticles constructed in accordance with the process of claim 1. JOHN MACK Primary Examiner 5 References Cited W. I. SOLOMON, AssistantExaminer UNITED STATES PATENTS 2,315,568 4/1943 Wernlund 204-145 R2,092,130 9/1937 Lyons, Jr. 20434 10 20434, 145 R

